Active matrix display devices

ABSTRACT

In an active matrix display device, particularly an AMLCD, having an array of display pixels ( 8 ) and comprising pixel electrodes ( 16 ), associated switches ( 22 ), and address lines ( 18, 20 ) carried on a first substrate ( 10 ), a common electrode ( 26 ) carried on a second substrate ( 12 ), a drive circuit ( 40, 80 ) carried on the first substrate ( 10 ) including at least one conductor line ( 96 ) providing a drive voltage for the common electrode ( 26 ) and to which the common electrode ( 26 ) is connected ( 92 ), the common electrode ( 26 ) on the second substrate is utilised to provide also electrical connection between the one conductor line ( 96 ) and at least one other circuit element ( 37 ) carried on the first substrate ( 10 ). The use of the common electrode in this way assists in avoiding problems due to resistances of connections formed on the first substrate. The connection to a storage capacitor line ( 37, 36 ) may benefit especially. The common electrode ( 26 ) may be connected to the circuit elements on the first substrate via contact material ( 92, 100 ) extending across the gap between the two substrates.

This invention relates to an active matrix display device comprisingfirst and second substrates, electro-optical material disposed betweenthe first and second substrates, an array of display pixels comprisingpicture element electrodes and associated switches carried together withsets of address conductors on the first substrate, and a commonelectrode carried on the second substrate, each picture elementelectrode together with an overlying portion of the common electrode andthe electro-optical material therebetween defining a pixel, and drivemeans connected to the sets of address conductors for applying drivesignals to the array of pixels.

A common display device of this form is an AMLCD (active matrix liquidcrystal display). A typical example of such is described in U.S. Pat.No. 5,130,829, the contents of which are incorporated herein asreference material. In this device an array of display pixels isprovided, arranged in rows and columns, with each pixel comprising anelectro-optic cell, constituting a picture element, formed by LCmaterial sandwiched between a picture element electrode and part of acounter electrode common to all pixels, and an associated thin filmtransistor (TFT).

Display devices of this type are commonly used in monitors, TVs, lap-topcomputers, PDAs, and mobile phones for example.

The TFTs of the pixels typically comprise amorphous silicon (a-Si) typeTFTs, or polycrystalline or microcrystalline silicon type TFTs formed onone substrate of the device, usually of glass, together with a pictureelement electrodes and sets of address conductors using well-known thinfilm processing technology, typically involving the deposition andpatterning of various conducting, insulating, and semi-conducting layersusing for example CVD and photolithographic techniques. In the case ofdevices employing a-Si TFTs, the driving circuitry, for example the rowdriver circuitry providing selection (scanning) signals to a set of rowaddress conductors, is usually provided in the form of one or moresilicon ICs. These can be provided on a PCB separate to the displaydevice substrates with, for example, a foil connector being used toconnect the outputs of the ICs to respective address conductors. It isknown also to mount the driver ICs on flexible foils connected to theaddress conductors. This can lead to cost reductions but a PCB isusually still employed to provide low resistance power lines andinterconnections between the ICs. The driver ICs can instead be mounteddirectly on one substrate of the device, for example using COG (Chip OnGlass) techniques, alongside one or more edges of the pixel array. Whenusing polycrystalline or microcrystalline silicon TFTs for the pixelsthe driver circuitry may conveniently be integrated on the device'ssubstrate carrying the active matrix circuit at one or more edges byforming the circuitry from thin film circuit elements and fabricatingthese elements simultaneously with the TFTs and other components of thepixel array from common deposited layers. In these two latterapproaches, however, where the driver circuits are provided on asubstrate of the device, problems can arise because of the higherresistances of connections on a glass substrate compared with thoseformed on a PCB. These problems can be particularly noticeable incomparatively large area AMLCDs, for example with a 20 inch diagonal orgreater. Voltages supplied to certain components in the pixel array needto be well defined and stable other wise the quality of the imagesproduced can be degraded. Connection resistances can affect suchvoltages, particularly in lines carrying larger currents, and may leadto visible artefacts in the display image. For example, verticalcross-talk effects, or longer range non-uniformity problems such as gatevoltages changes to the address TFTs of the pixels causing flicker inthe display image may result. Also, variation in reference voltagelevels over the array can produce dark and light banding effects fromthe top to the bottom of the display image.

One particular feature which may be affected seriously in this wayleading to unwanted display artefacts is the connection to pixel storagecapacitor lines. Storage capacitors are commonly used in the pixels ofan AMLCD to assist in maintaining a desired voltage across the pictureelement, and hence its display output, in the period following itsaddressing until it is next addressed, generally corresponding to aframe period. The storage capacitor of a pixel is connected at its oneside to the node between the TFT drain and the picture element electrodeand at its other side to a reference potential which may be a rowaddress conductor (gate line) associated with an adjacent (preceding orsucceeding) row of pixels, or alternatively to an auxiliary conductorextending in the row direction and shared by all pixels in the same row.These auxiliary conductors are interconnected with one another andconstitute a capacitor line. The introduction of resistances as low as,for example 5 to 10 ohms in the connection of this capacitor line canproduce visible artefacts in the display image. An electrical connectionformed on a glass substrate can typically result in a resistance of upto 100 ohms, and may thus be unacceptable.

According to the present invention there is provided an active matrixdisplay device comprising first and second substrates, electro-opticalmaterial disposed between the first and second substrates, an array ofdisplay pixels comprising picture element electrodes and associatedswitches carried together with sets of address lines on the firstsubstrate, and a common electrode carried on the second substrate, eachpicture element electrode together with an overlying portion of thecommon electrode and the electro-optical material therebetween defininga pixel, drive means connected to the sets of address conductors forapplying drive signals to the array of pixels, the drive meanscomprising a drive circuit which is carried on the first substrate andincludes conductor lines, the common electrode on the second substratebeing connected electrically to at least one conductor line on the firstsubstrate that provides a drive voltage for the common electrode, andthe common electrode on the second substrate being utilised to provideelectrical connection between the one conductor line and at least oneother circuit element carried on the first substrate.

The drive circuit may be in the form of one or more ICs which aremounted on the first substrate, at a peripheral edge region outside thepixel array area or may comprise thin film circuitry integrated on thefirst substrate, again at a peripheral edge region, fabricated in knownmanner simultaneously with the active matrix circuit associated with thepixels and from common deposited thin film layers. As a furtheralternative, the drive circuit may comprise one or more ICs mounted on afoil connector that is attached to, and supported by, the firstsubstrate with its conductive tracks or lines being connected to thecircuitry carried directly on the substrate. This may be advantageous inenabling defective ICs to be replaced.

The use of the common electrode for electrical connection purposes inthis manner is beneficial to avoiding the aforementioned connectionresistance problems. The common electrode, normally comprising atransparent electrically conductive material such as ITO, has arelatively low sheet resistance. Typically, the sheet resistance may beas low as a few ohms in the case where the ITO layer is backed by, andelectrically contacts, a metallic black mask grid. Such a grid ispresent in most AMLCDs on the common electrode substrate, its primaryfunction being to provide a light opaque border around the individualpicture elements to prevent optical cross-talk, and in some cases alsoto shield the pixel TFTs from light.

In an AMLCD the common electrode on one substrate is normally connectedto a voltage bias source on the other substrate by means of one or moreconnections, so-called transfer contacts, formed for example by a silverpaste, and bridging the gap between the two substrates outside thedisplay area, for example at the edge of one substrate. In a preferredembodiment of the invention, electrical connections between the commonelectrode on the second substrate and the conductor line on the firstsubstrate and between the common electrode and an electrical contactarea on the first substrate associated with the other circuit element onthe first substrate are provided in similar fashion, i.e using anelectrically conductive material such as silver paste bridging the gapbetween the two substrates. Preferably, a plurality of spaced bridgingconnections between the two substrates are provided, for examplearranged along opposing edges of the second substrate.

In a particular embodiment, the other circuit element comprises thestorage capacitor line, and in this case a capacitor connection lineconnecting together the individual storage capacitor row line portionsof the pixel rows provided on the first substrate at one side of thearray, for example adjacent an edge of the second substrate, or at eachof two opposing sides of the array, and this connection line providescontact areas for the bridging connections. The facility forinterconnection through the common electrode may also be used toadvantage in supplying other circuit elements on the active plateneeding a corresponding voltage level.

Although AMLCDs are of particular interest, the device may useelectro-optical materials other than liquid crystal crystal materials,such as electrophoretic or electrochromic materials for example.

An embodiment of active matrix display device, in particular an AMLCD,in accordance with the invention will now be described, by way ofexample, with reference to the accompanying schematic drawings, inwhich:—

FIG. 1 shows a highly simplified cross-sectional view through part of anembodiment of AMLCD according to the invention;

FIG. 2 illustrates the equivalent electrical circuit of the AMLCD ofFIG. 1;

FIG. 3 is a plan view of an example AMLCD illustrating a knownarrangement of certain components; and

FIG. 4 is a plan view of the embodiment of AMLCD of FIG. 1 illustratingcertain features and components of the device.

It will be appreciated that the Figures are all merely schematic and arenot drawn to scale. The same reference numbers are used throughout thefigures to denote the same or similar parts.

Referring to FIG. 1, the AMLCD is of generally conventional structurehaving a matrix array of display pixels 8 and comprises first and secondspaced substrates 10 and 12, typically of glass, between which a layerof liquid crystal material 14 is disposed. Only a relatively small partof the structure is shown in the Figure. The substrate 10, commonlyreferred to as the active plate, carries a row and column array ofindividual picture element electrodes 16 located adjacent respectiveintersections of crossing sets of row and column address conductors 18and 20, only the column address conductors 20 being visible in theFigure. Each picture element electrode 16 is connected to a respectiveswitching device in the form of a TFT (thin film transistor) 22 (notshown in FIG. 1) comprising an amorphous silicon or polycrystallinesilicon type FET. The active matrix circuitry comprising the electrodes16, address conductors 18, 20, and the TFTs 22 on the substrate 10 iscovered by a LC (liquid crystal) orientation layer 24.

The other substrate 12, commonly referred to as the passive plate,carries a continuous common, or counter, electrode 26 of transparentelectrically conductive material, such as ITO, which is co-extensivewith the array of picture element electrodes 16 and separated from theliquid crystal layer 14 by an LC orientation layer 28. The commonelectrode 26 overlies a structure comprising an array of red, green andblue colour filter elements 30 aligned with respective electrodes 16 ofthe array of electrodes 16, and a light opaque black mask layer 31, inthe form of a grid, portions of which extend between, and separate,individual colour filter elements 30 in known manner.

Each picture element electrode 16 together with a respective overlyingportion of the common electrode 26, a respective colour filter element30 and LC material therebetween defines a picture element which togetherwith its associated TFT 22 constitutes a display pixel 8.

The two substrates 10 and 12 are maintained at a desired separation byspacer elements (not shown) and sealed together around the periphery ofthe pixel array to contain the LC material by a seal 32 which alsodefines the boundary of the pixel array.

Referring now to FIG. 2, which shows the equivalent circuit of thedisplay device, each picture element electrode 16 is connected to thedrain of its associated TFT 22. The gates of the TFTs 22 of all thepixels 8 in the same row are connected to a respective row addressconductor 18. The sources of the TFTs 22 of all the pixels 8 in the samecolumn are connected to a respective column address conductor 20. Eachdisplay pixel 8 further includes, in known manner, a storage capacitor35 connected at one side to the node between its picture elementelectrode 16 and the TFT 22 and at its other side to a capacitor line 36shared by all the pixels in the same row and provided in the form of anauxiliary row conductor extending in the row direction. The capacitorrow lines 36 associated with all pixel rows are connected together atboth ends outside the area of the pixel array by capacitor connectionlines 37 extending along opposing sides of the array. A suitablereference potential source is connected to these lines in operation ofthe device to provide a desired operating voltage bias to the storagecapacitors.

The device is operated in generally conventional manner. Briefly, thesets of row and column address conductors 18 and 20 are respectivelyconnected at their ends to row and column driver circuits 40 and 42. Thepixel array is driven on a row at a time basis by means of the rowdriver circuit 40, comprising for example a digital shift registercircuit, scanning the row address conductors 18 sequentially with aselection (gating) pulse signal so as to turn on the TFTs 22 of thepixels 8 of each row in turn in a respective row address period. Thecolumn driver circuit 42 supplies data (video) signals to the columnaddress conductors 20 for each row of pixels in turn as appropriate andin synchronism with the row selection signals whereby the pictureelements in each row are driven according the level of the applied datasignal. Using one row at a time addressing, all TFTs 22 of the addressedrow are switched on by the selection signal on the associated rowconductor 18 for a period corresponding to the duration of that signalduring which time the data signals are transferred from the columnconductors 20 to charge up the picture element electrodes 16 and theirassociated storage capacitors 35. Upon termination of the selectionsignal, the TFTs of the row of pixels are turned off to isolate thepicture elements from the column conductors 20 and ensuring the appliedcharge is stored in the picture elements and storage capacitors untilthe pixels are next addressed. All rows of pixels are addressed in turnin this manner in a frame period to produce a complete display image andrepeatedly addressed in similar fashion in successive frame periods.Each picture element serves to modulate light passing through thepicture element according to the applied data signal which determinesits transmission characteristics. The device may be operable in atransmissive mode, in which the picture element electrodes 16 and thecommon electrode 26 are all formed of transparent conductive material orin a reflective mode in which either the common electrode or the pictureelement electrodes are formed of light reflective material.

Timed operation of row driver circuit 40 is controlled by a timing andcontrol circuit 44 which is also responsible for controlling the timingof the operation of the column driver circuit 42. The column drivercircuit 42 is of any conventional kind and may be of analogue or digitaltype. In the former case it may, for example, comprise one or more shiftregister/sample and hold circuits with the timing and control circuit 44supplying thereto video signal and timing pulses, in synchronism withrow scanning, to provide appropriate serial to parallel conversion.

During operation of the device, the common electrode 26 and thecapacitor connection line 37 are held at a constant reference potential,for example, ground. Alternatively, and in accordance with known driveschemes, the voltage on the electrode 26 may be modulated, in order toreduce the data signal voltage range necessary from the column drivercircuit 42, with the voltage of the capacitor connection line 37changing in corresponding fashion. In this case, the waveform applied tothe row address conductors 18 is modified such that the non-selection,gate off, level is similarly modulated by a corresponding amount toensure that the address TFTs 22 of the pixels are held fully off in theintervals between selection and to avoid unwanted capacitive couplingeffects.

From the foregoing, It will be appreciated that the generalconstructional and operational aspects of the device follow knownpractice and consequently have not been described in great detail.Further details in these respects may be obtained from U.S. Pat. No.5,130,829 for example.

FIG. 3 illustrates a known manner in which the row and column drivercircuits 40 and 42 may be provided. In this, the circuits each comprisea set of individual silicon ICs, or chips, 80, 90, mounted on respectivefoil connectors 82 comprising a flexible film carrying conductive trackswhich contact respective ones of the chip outputs. The substrate 12 isphysically smaller than the substrate 10 and mounted to the substrate 10so as to leave peripheral edge regions of the substrate 10 exposed. Thefoil connectors 82 are bonded at their one side to these edge regions ofthe substrate 10, where the tracks electrically connect with respectiveextensions of the address conductors 18, 20, and at their other side toa PCB (printed circuit board), 84, 86 respectively for the set of rowdriver chips 80 and the set of column driver chips 90. The PCBs 84 and86 each carry low resistance conductor lines associated with the set ofchips such as power rails for voltage distribution which need to be lowresistance to avoid voltage drops when currents flow in them, and timingsignal lines. An electrical connection from the substrate 10 to thecommon electrode 26 carried on the second substrate 12 is achieved byone or more transfer contacts extending between the two substratesoutside the seal line, and adjacent the edge of the substrate 12, asshown at 92 in FIG. 1. In the FIG. 3 arrangement two transfer contacts,92, are provided at adjacent corners of the substrate 12. The transfercontacts typically comprise silver paste material contacting on thesubstrate 12 an extension of the common electrode and on the substrate10 a conductive track, carrying the reference potential for the commonelectrode. A disadvantage of this kind of arrangement is that it can beexpensive to implement. Moreover, it is not especially suited toproducing a compact display device in view of the use of peripheral foilconnectors and PCBs.

In an alternative known arrangement, the chips 80 constituting the rowdriver circuit are instead mounted directly on an edge region of thesubstrate 10 employing, for example, COG technology and using conductortracks formed on the substrate 10 to provide the interconnectionfunction of the PCB 84, thereby eliminating the need for that PCB. Thechips 90 constituting the column driver circuit may be mounted on anedge of the substrate 10 in similar manner.

FIG. 4 shows a plan schematic view of the embodiment of AMLCD accordingto the invention. In this particular example amorphous silicon type TFTsare used in the display pixels and the row and column driver circuits 40and 42 each comprise a plurality of chips (ICs) 80, 90 arranged mutuallyspaced in a line. The chips 80 and 90 are mounted on respective edgeregions of the substrate 10 outside the pixel array area and the sealline and along respective adjacent sides of the pixel array. The chipsare mounted on the substrate using any suitable known technique, forexample COG, with individual output terminals of the chips electricallycontacting respective extensions (not shown) of the sets of row andcolumn address conductors 18 and 20 is known fashion. Electricallyconductive lines interconnecting the chips are formed on the surface ofthe substrate 10 extending along the edge regions which perform the samegeneral function as the tracks of the PCBs in the above—described knownarrangement in supplying power and timing signals, and, for the columndriver chips 90 video data signals. Only one such track, indicated at95, is shown in FIG. 4 for simplicity, the track 95 extending alongsidethe line of row driver chips 80. A transfer contact 92 is providedadjacent each corner of the substrate 12 connecting the common electrode26 at each of its four corners to driving voltage supply tracks 96 alsocarried on the substrate 10, each transfer contact 92 overlying andelectrically contacting a respective contact region of such a supplytrack.

Also arranged at spaced intervals alongside two opposing edges of thesubstrate 12 are additional bridging connections 100 extending betweenthe two substrates 10 and 12 and at the edge of the substrate 12 insimilar manner to the transfer contacts 92, there being a series ofthree additional contacts 100 provided on each side at opposing edges ofthe substrate 12 in the example illustrated although the number of suchcontacts can be varied. These bridging connections 100, formed similarlyto the transfer contacts 92, for example of silver paste, defineconnection points between the substrates 10 and 12. At the substrate 12they contact the common electrode 26, or an outwardly projectingextension thereof, and at the substrate 10 are arranged so as tooverlie, and electrically contact, portions of the capacitor connectionline 37 which for this purpose are arranged extending along therespective edge regions of the substrate 10 parallel to, andapproximately beneath, the opposing edges of the substrate 12. In theillustrated embodiment, the same conductive track is used to constitutethe capacitor connection line 37 and the track 96 supplying the drivevoltage to the transfer contacts 92 on each side of the array. Thetracks 96/37 are connected, via a foil connector 82, to a column driverPCB 86′ which carries the common electrode drive voltage generatingcircuit. Connections to external circuitry, providing for example powersupply, video signals, etc, is made from the PCB 86′, as shown at 102.

In this way, the common electrode 26 is utilised to provide, in additionto its usual function of serving as the second electrodes of the pictureelements of the array, a supply function for elements of the activematrix circuit on the substrate 10, in this case in providing a requiredpotential to the capacitor lines 36 and supplementing the connectionlines 37 provided on the substrate 10. This avoids problems which may beencountered in use due to nature of the lines 37 and their inherentresistance. Being formed from thin film conductive material on glassthese lines inevitably possess low conductivity compared with, forexample, the relatively much thicker conductive tracks of a PCB. Aspreviously explained, it is especially important for producing a highquality display image that the reference potential of the storagecapacitors 35 be well defined and stable. The introduction ofresistances as low as, for example, 5 ohms in the capacitor lines canresult in highly visible unwanted display artefacts and connections onthe glass substrate 10 can easily lead up to 100 ohms of connectionresistance. The common electrode 26 typically has an effective low sheetresistance, for example around just a few ohms in the case where themetallic black mask layer 31 is used which is in direct electricalcontact with the ITO material of the common electrode 26. By virtue ofthe fact that the common electrode 26 is connected to a voltage biassource via the transfer contacts 92 the presence of the plurality ofadditional bridging connections 100 enables a low resistance path toproviding the required voltage bias to the storage capacitor lines 36 onthe substrate 10.

To assist in minimising the sheet resistance of the common electrode 26and the connection resistance to the storage capacitor lines 26, theblack mask layer 31 is preferably made of a low resistance alloy, suchas aluminium, copper or silver alloys.

The same principle as described above to provide low resistanceinterconnect for the capacitor lines may be used for other components ofthe circuitry on the substrate 10 where a bias potential equivalent tothat of the common electrode is required. For example, such additionalinterconnects may be used for column driver circuit voltage levelreferencing purposes or in providing the appropriate, correspondinglevels of the waveforms applied to the row address conductors 18′ by therow driver circuit 40.

The use of the common electrode 26 in this way can eliminate the need toprovide a separate PCB in association with the row driver circuitry,shown at 84 in FIG. 3. The technique can also lead to a simplified, andphysically smaller, PCB associated with the column driver circuitry, asshown at 86′ in FIG. 4 where the common electrode 26 is utilised as wellto perform certain connection functions previously accomplished usingPCB conductor tracks. This is achieved by a set of further additionalbridging connections 100′, similar to contacts 100, being used betweenthe substrates 10 and 12 along the upper side edge of the substrate 12to this end.

The additional bridging connections 100 and 100′ are conveniently formedin the same manner and at the same time as the transfer contacts 92using any suitable known techniques for forming such transfer contacts,for example as described in U.S. Pat. No. 5,625,476, the contents ofwhich are incorporated herein as reference material.

It is envisaged that the driver circuit ICs need not be mounted directlyon the surface of the substrate 10. As an alternative, the driver ICs,for example the chips 80, may each be mounted on a foil that is carriedon, and supported by, the substrate 10 at a peripheral region with theconductive tracks of the foil connecting to the circuitry on thesubstrate. This allows for easy replacement of the ICs should defectsoccur.

Although in the above described embodiment silicon ICs are used for thedriver circuits, these circuits may instead be fully integrated on thesubstrate 10 using thin film component circuitry comprising TFTs,capacitors and interconnections, when using polycrystalline ormicrocrystalline silicon TFTs for the pixels 8. As is well known, rowand/or column driver circuits can be readily fabricated using thin filmtechnology similar to that used to produce the active matrix circuitry.Fabrication of the driver circuits simultaneously with the active matrixcircuitry, from common deposited thin film layers, leads to considerablecost savings.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the field of active matrixdisplay devices and component parts therefor and which may be usedinstead of or in addition to features already described herein.

1. An active matrix display device comprising first and secondsubstrates, electro-optical material disposed between the first andsecond substrates, an array of display pixels comprising picture elementelectrodes and associated switches carried together with sets of addresslines on the first substrate, and a common electrode carried on thesecond substrate, each picture element electrode together with anoverlying portion of the common electrode and the electro-opticalmaterial therebetween defining a pixel, drive means connected to thesets of address conductors for applying drive signals to the array ofpixels, the drive means comprising a drive circuit which is carried onthe first substrate and includes conductor lines, the common electrodeon the second substrate being connected electrically to at least oneconductor line on the first substrate that provides a drive voltage forthe common electrode, and the common electrode on the second substratebeing utilised to provide electrical connection between the oneconductor line and at least one other circuit element carried on thefirst substrate.
 2. A device according to claim 1, wherein the drivecircuit comprises at least one integrated circuit mounted on the firstsubstrate.
 3. A device according to claim 1, wherein the drive circuitcomprises thin film circuit elements integrated on the first substrate.4. A device according to claim 1, wherein the common electrode isconnected to the at least one other circuit element via at least oneconnection element extending between the first and second substratesadjacent an edge of the second substrate.
 5. A device according to claim1, wherein the display pixels include storage capacitors which areconnected at their one side to a capacitor connection line carried onthe first substrate and wherein the common electrode is utilised toprovide electrical connection between the one conductor line and thecapacitor connection line.
 6. A device according to claim 5, wherein thecapacitor connection line extends on the first substrate adjacent oneedge of the second substrate and connects together at one side of thearray a plurality of capacitor connection line row portions, each rowportion being connected to the storage capacitors of a respective row ofpixels, and wherein the common electrode is connected electrically withthe capacitor connection line at spaced locations along that edge of thesecond substrate.
 7. A device according to claim 6, wherein thecapacitor connection line also extends on the first substrate adjacentan opposing edge of the second substrate and connects together theplurality of capacitor connection row portions at the opposing side ofthe array, and wherein the common electrode is connected with thecapacitor connection line at spaced locations along the opposing edge ofthe second substrate.
 8. A device according to claim 6, wherein thecommon electrode is connected to the capacitor connection line viabridging connections extending between the first and second substratesarranged along a substantial part of the length of the edge of thesecond substrate.
 9. A device according to claim 8, wherein the bridgingconnections comprise conductive material disposed between the twosubstrates adjacent the edge of the second substrate.
 10. A deviceaccording to claim 1, wherein the common electrode is connected to theat least one conductive line on the first substrate via at least onebridging connection comprising conductive material disposed between thetwo substrates adjacent an edge of the second substrate.
 11. A deviceaccording to claim 10, wherein the common electrode is connected to theat least one conductive line via a plurality of bridging connectionsarranged adjacent corners of the second substrate.
 12. A deviceaccording to claim 1, wherein in the second substrate carries a metallicblack mask layer adjacent to, and in electrical contact with, the commonelectrode.
 13. A device according to claim 1, wherein theelectro-optical material comprises liquid crystal material.